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| United States Patent |
6,066,591
|
|
Murphy
, et al.
|
May 23, 2000
|
Reactive medium for purifying fluids
Abstract
A method for forming a reactive medium for removing homogenous and
heterogeneous impurities from a fluid is provided which includes forming a
uniform oxide layer on all exposed surfaces of a porous metal substrate,
depositing at least one layer of carbon on substantially all of the oxide
layer formed on the porous metal substrate, depositing a precursor metal
species on the carbon layer, and heating the porous metal substrate
containing the carbon layer to form active sites on the carbon layer,
wherein the active sites include at least partially deoxygenated metal
species chemically bonded to the carbon layer. Also provided is a reactive
medium for purifying a fluid, the reactive medium comprising a porous
inorganic substrate, a layer including carbon and an oxide layer between
the porous inorganic substrate and the layer including carbon. The
reactive medium further comprises active sites including at least
partially deoxygenated metal species, the active sites being bonded to the
layer including carbon.
| Inventors:
|
Murphy; William L. (Homer, NY);
Wilson; Bruce A. (Homer, NY)
|
| Assignee:
|
Pall Corporation (East Hills, NY)
|
| Appl. No.:
|
184349 |
| Filed:
|
November 2, 1998 |
| Current U.S. Class: |
502/417; 55/350.1; 55/524; 427/244; 427/245; 427/247; 427/249.1; 427/327; 502/4; 502/416 |
| Intern'l Class: |
B01J 020/02 |
| Field of Search: |
29/896.62,458,459,527.2
55/524,350.1
95/285,901
502/4,416,417
423/210
427/244,245,247,249,301,327,402
|
References Cited
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| 4613369 | Sep., 1986 | Koehler | 75/246.
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| 4613815 | Sep., 1986 | Christel, Jr. | 324/233.
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| 4713224 | Dec., 1987 | Tamhankar et al. | 423/219.
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| 4782226 | Nov., 1988 | Jeffries, Jr. et al. | 250/227.
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| 4789386 | Dec., 1988 | Vaughn et al. | 55/158.
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| 4822692 | Apr., 1989 | Koehler | 428/547.
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| 4853148 | Aug., 1989 | Tom et al. | 252/194.
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| 4867960 | Sep., 1989 | Tom | 423/488.
|
| 4906263 | Mar., 1990 | von Blucher et al. | 55/316.
|
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| 4921823 | May., 1990 | Furneaux et al. | 502/4.
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| 4938785 | Jul., 1990 | MacPherson, Jr. | 55/1.
|
| 4946592 | Aug., 1990 | Galaj et al. | 210/490.
|
| 4981501 | Jan., 1991 | Von Blucher et al. | 55/316.
|
| 5037791 | Aug., 1991 | Comolli et al. | 502/185.
|
| 5114447 | May., 1992 | Davis.
| |
| 5139747 | Aug., 1992 | Cato et al. | 422/122.
|
| 5149360 | Sep., 1992 | Koehler et al. | 75/228.
|
| 5196380 | Mar., 1993 | Shadman | 502/4.
|
| 5204075 | Apr., 1993 | Jain et al. | 423/219.
|
| 5262198 | Nov., 1993 | Wu et al. | 427/249.
|
| 5302356 | Apr., 1994 | Shadman et al.
| |
| 5456740 | Oct., 1995 | Snow et al. | 55/524.
|
| 5635148 | Jun., 1997 | Shadman.
| |
| 5637544 | Jun., 1997 | Shadman.
| |
| Foreign Patent Documents |
| 508145 | Dec., 1954 | CA.
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| 0083489 | Jul., 1983 | EP.
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| 0428052 | May., 1991 | EP.
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| 0596441 | May., 1994 | EP.
| |
| 2251351 | Jul., 1975 | FR | 210/506.
|
| 745439 | May., 1943 | DE.
| |
| 2906510 | Aug., 1979 | DE.
| |
| 50-6440 | Mar., 1975 | JP.
| |
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| |
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| |
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| |
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| |
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| |
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| |
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| |
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| |
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| |
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| |
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| |
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|
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| |
Other References
"The Preparation of and Characterization of Alumina Membranes with
Ultra-fine Pores", Journal of Material Science, pp. 1077-1088, 1994.
Leonaars et al., "Porous Alumina Membranes", Chemtech, pp. 560-564, Sep.
1986.
|
Primary Examiner: Dunn; Tom
Attorney, Agent or Firm: Leydig, Voit & Mayer
Parent Case Text
This application is a divisional of U.S. application Ser. No. 08/976,118
filed on Nov. 21, 1997, now U.S. Pat. No. 5,829,139 which is a
continuation of U.S. application Ser. No. 08/433,022 filed on May 3, 1995
and now abandoned, both of which are incorporated by reference in their
entireties.
Claims
What is claimed is:
1. A reactive medium for purifying a fluid comprising:
a porous metal substrate;
a layer including carbon;
an oxide layer between the porous metal substrate and the layer including
carbon; and
active sites including at least partially deoxygenated metal species, the
active sites being bonded to the layer including carbon.
2. The reactive medium of claim 1 wherein the oxide layer comprises
oxidized metal of the substrate.
3. The reactive medium of claim 2 wherein the oxide layer is disposed on
substantially all surfaces of the porous metal substrate.
4. The reactive medium of claim 3 wherein the oxide layer comprises iron
oxide.
5. The reactive medium of claim 4 wherein the metal comprises stainless
steel.
6. The reactive medium of claim 1, wherein the oxide layer is disposed on
substantially all surfaces of the porous metal substrate.
7. The reactive medium of claim 1 wherein the oxide layer is uniformly
disposed between the porous metal substrate and the carbon layer.
8. The reactive medium of claim 1 wherein the ratio of the total surface
area of the carbon coating to the total surface area of the metal
substrate is in the range from about 25 to about 65 m.sup.2 /m.sup.2.
9. The reactive medium of claim 1 wherein the partially deoxygenated metal
species is selected from the group consisting of alkali metals, manganese,
magnesium, and mixtures thereof.
10. The reactive medium of claim 9 wherein at least one of the deoxygenated
metal species includes sodium.
11. The reactive medium of claim 10 wherein the ratio of sodium metal to
surface area of carbon is the range from about 1.2 to about 1.6 mg
sodium/m.sup.2 carbon.
12. The reactive medium of claim 1 wherein the metal of the porous metal
substrate is selected from the group consisting of nickel, the Hastalloy
family of metals, the Monel family of metals, and stainless steel.
13. A reactive medium for purifying a fluid comprising:
a porous metal substrate;
an oxide layer disposed on substantially all surfaces of the porous metal
substrate;
at least one layer of carbon disposed on the oxide layer; and
active sites including at least partially deoxygenated metal species, the
active sites being bonded to the carbon layer and including at least one
of manganese, magnesium, and an alkali metal.
14. The reactive medium of claim 13 wherein the metal of the porous metal
substrate is selected from the group consisting of nickel, the Hastalloy
family of metals, the Monel family of metals, and stainless steel, and
wherein the oxide layer comprises oxidized metal of the substrate.
15. The reactive medium of claim 13 wherein the porous metal substrate
comprises stainless steel and the oxide layer comprises iron oxide.
16. The reactive medium of claim 15 wherein the iron oxide layer is
disposed on substantially all surfaces of the stainless steel substrate.
17. A reactive medium for purifying a fluid comprising:
a porous stainless steel substrate;
a layer including carbon;
an oxide layer between the porous stainless steel substrate and the layer
including carbon, the oxide layer comprising iron oxide and being disposed
substantially on all surfaces of the porous metal substrate; and
active sites including at least partially deoxygenated metal species, the
active sites being bonded to the layer including carbon.
18. The reactive medium of claim 17 wherein the partially deoxygenated
metal species is selected from the group consisting of alkaline metals,
manganese, and magnesium, and mixtures thereof.
Description
FIELD OF THE INVENTION
This invention relates to a filter assembly used for filtering fluids. More
particularly the present invention is directed to a filter assembly used
in the filtration and ultrapurification of gases and to a method of
filtering and purifying gases.
BACKGROUND OF THE INVENTION
Advances in all areas of modern technology have, to a large degree,
depended on the modification and development of new materials and the
purification of substances used as both reagents and as materials, in the
presence of which various processes are conducted. The purification of
many such substances, typically liquids and gases, has required the
removal of impurities which are either heterogeneous (such as particles
and macromolecules) or homogeneous (such as dissolved substances).
Typically, heterogeneous impurities are removed by filtration techniques
and devices in which the particles are physically retained by some sort of
perforate or porous medium. Other methods and purification techniques are
typically chosen to remove homogeneous impurities. Many of these
techniques involve the chemical modification and/or the affinity and
attraction of the homogeneous impurity and the resulting removal of such
material from the fluid.
In many areas of modern technology, the concentration of impurities above
several parts per million (ppm) cannot be tolerated and in certain
technologies, such as in the manufacture of semiconductor devices, the
concentration of impurities in both the substances used as reagents as
well as other materials, in the presence of which the processes are
conducted, can still be detrimental even at levels at or below several
parts per billion (ppb). For example, in many of the process gases
employed in manufacturing semiconductor devices, impurities such as
moisture, oxygen and organic compounds in even trace amounts can be
adsorbed on the semiconductor wafer, causing degradation of performance,
reduced manufacturing yield and adverse reliability.
In such applications, to remove homogeneous impurities from process gases,
various commercial purification techniques and purifiers which involve
physical adsorption and chemisorption of impurities or conversion of
impurities to other forms which can be adsorbed on a solid substrate are
employed. Most of these purification techniques employ a packed bed of
particles or expanded materials. Examples of such materials include
various resins (e.g., Nanochem.RTM. resins) and various alloys (e.g.,
Zr--V--Fe alloys). For purification by this technique, the gas stream is
passed through these packed beds and the impurities react with the
sorption material. While capable of removing impurities on the ppm level,
these purification materials often do not effectively filter trace
homogenous impurities, such as reactive gases, present at the ppb level.
Moreover, these packed beds tend to be ineffective when there is an abrupt
surge in the impurity level due to the inefficiency of the packed beds in
effecting contact between impurity molecules and the resin or alloy of the
bed material. In addition, because such materials themselves tend to
generate heterogeneous impurities because of mechanical motion and
attrition of particles of the packed bed material, they have limited
service lifetimes. Furthermore, these materials are typically not reusable
and often cannot be regenerated.
For the removal of particulate material, various porous ceramics have been
employed. For example, U.K. Patent No. 2,201,355 to Dahlquist et al.
employs a porous membrane for separating heterogeneous impurities from an
aqueous medium. The porous membrane comprises an outer support matrix
having through-passages and an inner layer lining the through-passages and
deposited on the outer support. A polymer, metal or ceramic is used as the
support matrix. The inner layer is a matrix of particles of aluminum
hydroxide, partially hydrated aluminum oxide, silicon dioxide or zirconium
dioxide. French Patent No. 2,251,351 describes a microporous ceramic
filter that includes a microporous ceramic support electrophoretically
coated with an oxide of Al, Si, Mg, Ti, Cr, Ni, Zr or Fe. U.S. Pat. No.
3,288,615 to Estes et al. describes a ceramic filter body that includes a
framework of one or more tectosilicates with a mineral species (e.g.,
aluminates and oxides) distributed throughout and filling the framework.
While these and other filtration and purification materials permit removal
of impurities with varying degrees of success from a gas stream during
operation, significant potential for introduction of contamination to the
system occurs during start-up periods. This is particularly true when the
filtration or purification device is placed into service in the process
stream. Thus, referring to FIG. 1, which illustrates in section a
conventional filter device used in gas process streams, a filter is
incorporated within a housing through which the gas stream flows. More
particularly, the device 1 includes a fluid purification filter 3, often a
reactive gas filter, disposed within the housing 5 having a fluid inlet 7
and fluid outlet 9 formed as fluid connectors at opposite ends of the
housing. Such an arrangement requires that all of the gas pass through the
fluid purification filter 3 before reaching the outlet 9. Prior to use,
caps, typically formed from a metal material, are placed over the inlet
and outlet connectors, 7 and 9, to keep dust, other particulate
contaminants, and in some instances, fluids, from entering the housing.
Immediately prior to use, the caps are removed so that the housing may be
inserted into the system. Devices with structures of the type shown in
FIG. 1, however, allow for the introduction of contaminants from a number
of sources to the system.
One source of contamination originates within the housing 5 between the
filter 3 and the inlet 7 from air or other fluid present in or flowing
into the upstream portion of the housing through the inlet 7 when the
device is installed in the process stream. In many instances, air,
components of air or impurities in air, are detrimental to the process
stream and the reactive gas filter is capable of removing the undesirable
components or impurities present in the air which would adversely affect
the process stream. Once the device is installed in the process stream and
flow begins to pass from the process stream through the housing, the air
initially present in or entering the housing through the inlet does not
introduce further impurities to the process stream since the stream passes
through the filter 3. It is, however, desirable to minimize the amount of
air, contaminants, etc. entering the device since the filter 3 has a
limited capacity to remove impurities. To minimize the amount of air
entering the process stream at the inlet 7 when the device is installed, a
valve, such as a poppet valve, could be placed within the inlet 7 and the
housing pressurized with the gas employed in the process stream while the
outlet of the housing is capped. However, while such an approach is
effective in some instances to flush a filter device such that it may be
used immediately after the device is capped, in other instances there is
the potential for introduction of a second source of contamination into
the process stream. Thus, depending upon the location and climatic
conditions where the filter device is pressurized and the location and
atmospheric pressure where the filter device is used, the gas in the
downstream side of the filter housing, between the filter 3 and the outlet
9, may be displaced by atmospheric air or other fluid in the vicinity of
the housing stream once the cap is removed from the outlet and the filter
housing is connected in the fluid stream. Although in some instances it
may be possible to flush the filter device with an inert gas or the gas
employed in the process stream at the time the device is placed into
service, this requires one or more additional steps.
With current technologies, in addition to sources of contamination
introduced from the ambient environment of the filtration/purification
assembly when the device or assembly is placed into service, contamination
may be introduced by diffusion of atmospheric contamination located in the
vicinity of the inlet and particularly the outlet of the housing. While
the concentration of such contaminants may be very small, in particular
applications even trace amounts of such contaminants may be detrimental to
the processes and products involved.
SUMMARY OF THE INVENTION
The present invention is directed to a filtration/purification assembly for
use in fluid streams, preferably gas streams, which is capable of removing
contaminants from the fluid stream and which avoids introduction of
contaminants to the fluid stream and fluid assembly at the time the filter
assembly is placed in operation. The filtration/purification assembly of
the present invention is capable of reducing both homogeneous and
heterogeneous contaminants to concentrations in the ppb to parts per
trillion (ppt) range. The present invention is also capable of minimizing
or eliminating contamination from entering the filtration/purification
assembly, and hence the fluid stream, at the time the
filtration/purification assembly is placed into service and start-up of
the fluid stream is commenced. This results in greater capacity and longer
service life for the assembly.
The filtration/purification assembly of the present invention which
provides such filtration and purification levels includes a housing having
a fluid inlet and a fluid outlet. Disposed within the housing between the
fluid inlet and fluid outlet, and extending across a fluid flow path
provided therebetween is a fluid permeable, reactive filter medium,
preferably a gas permeable reactive medium. At both the fluid inlet and
fluid outlet are disposed barrier members. As defined with respect to the
present invention, "barrier member" is a member which prevents particulate
and fluid contaminants from entering the inlet or outlet, and thereby the
housing. In some instances the member may be used to prevent entry of all
fluids to the housing, except under controlled conditions. The barrier
member may be a medium or membrane or, a device, such as a valve. It is
preferred that at the fluid outlet the barrier member take the form of a
medium, preferably a type of a reactive medium, similar to the fluid
permeable, reactive filter medium disposed intermediate the fluid inlet
and fluid outlet and employed as the main means of removing the bulk of
contaminants entering the housing. At the inlet, it is preferred that the
barrier member be either a valve, such as a poppet valve, or a medium,
such as a reactive medium, also similar to the fluid permeable, reactive
filter medium used in the interior of the housing.
The reactive medium employed in the present invention includes a porous
inorganic or organic substrate, preferably a metallic substrate having
exposed surfaces, and including at least one carbon layer deposited on
substantially all of the exposed surfaces of the substrate. The carbon
layer is modified to present active sites which include at least partially
or substantially deoxygenated metal species chemically bonded to the
carbon layer. As used herein, the terms "rat least partially deoxygenated
metal species" and "substantially deoxygenated metal species" refer to
metals which have been reduced so as to be chemically bound to less than
the stoichiometric amount of oxygen.
The device according to the present invention is highly effective in
removing a variety of contaminants, both heterogeneous and homogeneous,
from fluids, particularly gases. Accordingly, the purification system of
the present invention may find application in a variety of processes. For
example, the filter assemblies of this invention may be employed in the
removal of gaseous and particulate contaminants in gas or liquid source
streams used in electronics manufacture. Depending upon the selection of
the partially deoxygenated metal species which forms part of the reactive
medium, trace impurities such as oxygen, water, carbon monoxide, carbon
dioxide, methane and other hydrocarbons may be removed from a gas stream
(e.g., a stream of an inert or noble gas such as nitrogen or argon). In
addition to inert gas streams, the present invention may be employed to
remove moisture from streams of reactive gases such as oxygen, silanes,
hydrogen chloride and hydrogen bromide.
The device of the present invention is capable of reducing the total
concentration of impurities, both heterogenous and homogeneous, to no more
than about 10 ppb, preferably to no more than about 1 ppb, and in some
instances below 10 ppt. The device according to the present invention is
capable of reducing, in an inert gas stream, the concentration of water,
oxygen or carbon monoxide to no more than about 10 ppt, methane and other
hydrocarbons to a concentration of no more than about 10 ppt and
preferably to no more than about 2 ppt and carbon monoxide to no more than
about 12 ppt.
As indicated above, in addition to purifying gases, once inserted into a
fluid stream, the filtration/purification assembly of the present
invention is also able to remove impurities from ambient fluids,
particularly gases, such as air, at the time the device is placed into
service. This is true independent of such factors as whether the device is
pressurized at the time of manufacture or prior to use, where the device
was manufactured or what ambient conditions existed at the time of
manufacture or use. Thus, a barrier member located at the inlet of the
assembly will minimize or substantially eliminate entry of contaminants to
the housing. In most instances, there is little to no chance of
contaminants being present in the gaseous volume in the portion of the
housing between the housing inlet, or the upstream barrier member, and the
reactive filter medium, known as the "upstream capture volume". However,
even if trace quantities of contaminants are present, once the device of
the present invention is connected in line to the fluid stream at both the
inlet and outlet provided in the housing, initiation of fluid flow will
cause fluid in the upstream capture volume to pass through the reactive
medium, thereby removing any homogeneous and heterogeneous contaminants in
the upstream capture volume. Likewise, particularly when the barrier
member at the outlet end of the housing is formed from the same or a
similar material as the reactive filter medium, contaminants present in
the "downstream capture volume" (the volume of gas in the portion of the
housing defined by the walls of the housing, the reactive medium and the
outlet end of the housing or barrier member located at the outlet) will be
substantially retained by the reactive filter medium in the outlet when
the filtration/purification assembly is placed in a fluid stream. As a
result, the barrier members employed in the invention increase the
capacity and extend the service life.
In addition to being capable of reuse and long service life, the reactive
filter device of the present invention is also capable of being
regenerated.
As suggested above, the present invention is also directed to a method for
filtering and purifying fluids, particularly gases, to remove
heterogeneous and homogeneous impurities to levels of several hundred ppt
and in some instances down to below the 10 ppt level. Such a method
involves placing the assembly in a fluid stream, such as a gas stream, and
providing fluid communication to the inlet of the housing such that the
fluid passes through the reactive medium which retains the contaminants
present in the fluid stream and provides purified effluent.
The present invention also relates to a method of preparing a reactive
filtration/purification medium and to a method of forming a reactive
filtration/purification assembly. The medium and the reactive
filtration/purification assembly itself produce higher purity fluid
streams and demonstrate an increased capacity to remove impurities
compared to known systems. The method of forming the improved reactive
medium according to the present invention involves forming a uniform oxide
layer on all exposed surfaces of a porous organic or inorganic substrate,
preferably a metal substrate. Thereafter, at least one layer of carbon is
deposited on substantially all of the oxide layer formed on the porous
metal substrate and afterwards a precursor metal species is deposited on
the carbon layer. Subsequently, the porous metal substrate containing the
carbon layer is heated to form active sites on the carbon layer such that
the active sites include at least partially deoxygenated metal species
chemically bonded to the carbon layer. In forming the
filtration/purification assembly, the porous substrate is disposed in a
housing section prior to formation of the uniform oxide layer and
subsequent treatments.
The uniform oxide layer of the metal substrate is preferably formed by
initially contacting all of the exposed metal surfaces of the metal
substrate with a reducing agent. Thereafter, the treated metal surfaces
are contacted with an oxidizing agent at an elevated temperature.
According to the preferred method of forming the reactive medium, the
uniform oxide coating provided on the exposed metal surfaces of the
substrate serves to initiate the deposition of carbon and to achieve
improved anchoring or bonding of the carbon to the substrate. This
ultimately results in a greater number of active sites on the medium.
The present invention further relates to a reactive medium for purifying
fluids. The reactive medium comprises a porous metal substrate, a layer
including carbon, and an oxide layer between the porous inorganic
substrate and the layer including carbon. The reactive medium further
comprises active sites including at least partially deoxygenated metal
species, the active sites being bonded to the layer including carbon.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view of a conventional gas filtration device.
FIG. 2 is a sectional view illustrating an embodiment of the present
invention.
FIG. 3 is a sectional view illustrating another embodiment of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
An embodiment of the present invention is illustrated in cross-section in
FIG. 2. This embodiment includes a reactive filtration/purification
assembly 11 which incorporates a reactive filter medium 13 disposed within
a housing 15 transverse to the fluid flow direction, as indicated by an
arrow in FIG. 2. The reactive filter medium 13 is so positioned and
affixed within the housing 15 that all fluid entering the housing must
flow through the reactive filter medium. At one end of the housing 15 is
provided a fluid inlet 17 and at the opposite end of the housing is
provided a fluid outlet 19. While the housing may have any configuration,
provided that the reactive filter medium extends completely across the
fluid flow path in the housing between the fluid inlet 17 and the outlet
19, a preferred configuration is a cylindrical housing 15, such as that
illustrated in FIG. 2, in which a circular reactive filter medium 13 is
placed centrally or coaxially within the housing. Within the inlet 17 and
in fluid communication with the interior of the housing, particularly the
upstream capture volume 25, is a first barrier member 21. Located within
the outlet 19, preferably at or proximate the opening of the outlet at
which effluent fluid emerges from the assembly, and in the fluid
communication with the housing, particularly the downstream capture
volume, is located a second barrier member 23.
The function of both the first barrier member 21 and the second barrier
member 23 is to minimize or substantially eliminate entry of either
heterogeneous or homogeneous contaminants to the filter housing 15 prior
to placement of the assembly into service in the fluid stream, and thereby
increase the capacity and service life of the assembly. Typically, such
barrier member takes the form of either a valve or a type of reactive
filter medium similar to that used to remove the bulk contaminants from
the fluid stream passing through the housing (i.e., reactive filter medium
13). The embodiment illustrated in FIG. 2 includes a valve, in this
instance a one-way valve such as a poppet valve at the inlet. A type of
reactive filter medium is located at the outlet. The embodiment
illustrated in FIG. 3 includes a type of reactive filter medium as the
barrier member at both the inlet and outlet of the filtration/purification
assembly device.
Preferred as the barrier member 23 (and 23'), located at the downstream
opening of the outlet 19, is another or second reactive filter medium,
similar in some respects (as discussed below) to the first or main
reactive filter medium 13.
The barrier member 21 (or 21'), provided at the inlet 17, is a poppet valve
in the embodiment of FIG. 2 (and a reactive filter medium in the
embodiment illustrated in FIG. 3). Thus, in the embodiment illustrated in
FIG. 3, the barrier members 21' and 23' are of the same type, i.e.
reactive filter media which are similar to the fluid permeable, reactive
filter medium 13, located within the housing. It is also preferable in the
embodiment shown in FIG. 3, that the materials from which barrier members
21' and 23' are made are the same. Depending on the particular
application, the barrier members 21' and 23' may have the same or
different fluid flow characteristics as each other.
It is preferred to locate the barrier member 23 (or 23') in the fluid
outlet 19 proximate the downstream opening to minimize or substantially
eliminate the volume between the tip of the outlet and the barrier member
23 (or 23') to prevent ambient gases from entering the housing 15 and the
fluid stream as the assembly is being inserted into the line. It is also
preferred, but is not as important as with the downstream barrier member
23 (23'), to place the upstream barrier member 21 (or 21') near the inlet
17 since any fluid flowing into the housing will be filtered and purified
by the main reactive filter medium 13 before reaching other parts of the
fluid stream.
Other than those reactive filter media used as barrier members, the
purification system of the present invention has been described thus far
and illustrated in FIGS. 2 and 3 as having a single layer or sheet of
reactive filter medium 13 fixedly positioned within the housing between
the inlet 17 and the outlet 19. While this embodiment is suitable for most
applications, and is preferred because of ease of manufacturing and other
reasons, there may be instances in which a plurality of sheets of reactive
filter medium, in spaced relationship to one another, may be substituted
for the single sheet reactive filter medium. Thus, in some instances two
or more sheets of reactive medium having the same or different fluid flow
or chemical composition characteristics, may be arranged in series within
the body of the housing. In such instances, the overall pressure drop
across the serially arranged sheets must meet the requirements of the
particular application in which the assembly is employed.
In a similar manner, rather than the single layer or sheet of reactive
medium 21' employed as the barrier member in an embodiment of the type
illustrated in FIG. 3, a plurality of sheets, preferably two, of reactive
medium may be substituted for the single sheet illustrated.
In addition, upstream of the barrier member employed in the inlet,
typically the reactive filter medium(s) 21', a filter element (not shown)
may be provided to trap larger particulate material. Such filter element
may be formed from a material which is inert under coating, purification
and filtration conditions and which will not contain or form volatile
substances. The material should be of an appropriate porosity to trap
particulate material slightly larger than would be retained by any
reactive filter medium used in the device. Typically organic polymeric and
metal materials may be used in the upstream filter. The filter element may
be a surface filter or a depth filter, appropriate to the situation. Such
an embodiment is not expected to find widespread application in the
electronics industry, particularly in the semiconductor industry, since
the fluids employed therein may contain some particulate material but of
relatively low proportions and of a very fine nature.
The preferred reactive filter media of the present invention are similar to
those of the type described in U.S. Pat. No. 5,637,544, specifically
incorporated herein by reference. Such reactive filter media include at
least one substrate layer, preferably an inorganic substrate layer, having
a plurality of pores therein, at least one layer of carbon deposited on
each substrate layer and coating the pores thereof, and at least one
reactive layer of a metal in reduced form (i.e., a partially or
substantially deoxygenated metal species) which is chemically bonded to
the carbon layer and is capable of reacting with impurities in a gas. As
indicated above, reactive media filters are different than most types of
existing filters in that in addition to being able to remove heterogeneous
impurities, they are also capable of significantly reducing homogeneous
impurities by interacting with and removing trace impurities.
When used as the main, fluid permeable, reactive filter medium 13, between
the inlet and outlet of the housing, typically the medium constitutes a
composite medium in which a plurality of coated substrate layers contact
each other. The number of substrate layers employed for the reactive
filter medium 13 depends on factors such as pressure drop across the
reactive filter medium or matrix of media, particle removal efficiency and
total contaminant removal capacity. Typically the number depends on the
surface area and fluid flux necessary to achieve a satisfactory separation
layer. When used as the barrier member 21', 23 or 23', the number of
layers is generally fewer than the number used in the reactive filter
medium 13 and is typically one to several layers.
With thinner reactive medium filters or barrier members, particularly the
latter, when the medium tends to be relatively non-self-supporting, it may
be desirable to include a foraminous metal support within or surrounding
the barrier member or filter member. Typically this is a very open pore
material, such as a metal mesh, particularly a stainless steel mesh.
Suitable would be any open pore fibrous or powder material or mesh in
various configurations. Preferred is a K-mesh material, which is a fine
wire stainless steel mesh. Most preferred is a substrate such as that
available from Pall Corporation under the tradename of "ULTRAMET-L" which
includes fine stainless steel fibers in a support envelope of a fine
stainless steel mesh screen.
The inorganic substrate is preferably a porous metal substrate. Of the
metals employed as the substrate, high alloys or corrosion resistant
metals and alloys, such as stainless steel, are preferred. Exemplary of
suitable metals are nickel, the Hastalloy family of metals, the Monel
family of metals, 316 L stainless steel and other similar alloys. The
metallic substrates are formed from metal particulate, such as metal
powder, metal fibers or combinations thereof. Preferred are substrates
formed from metal fibers. Suitable for use in the invention are fibers
having diameters of less than about 0.1 to about 12 microns, preferably
about 0.1 to about 4 microns.
Prior to coating, the metal substrates of this invention have a pore rating
of about 0.5 microns to about 10 microns, preferably about 2 microns to
about 6 microns. The void fraction of the uncoated metal substrate ranges
from about 50 to about 95% voids, preferably about 60 to about 80% voids.
The thickness of the carbon layer deposited on the porous substrate ranges
preferably from about 50.ANG. to about 1000.ANG.. While coating the
substrate with carbon may reduce the pore rating and concomitantly
increase the .DELTA.P across the medium, the particular application of the
assembly determines whether the .DELTA.P adversely affects the use of the
assembly.
The carbon layer is modified to present active sites which include at least
one metal species in reduced form chemically bonded to the carbon layer.
The metal species is capable of reacting or otherwise interacting with the
impurities of the gas stream in its reduced form. The term "reduced form"
relates to the oxidation state of the metal in that oxygen is chemically
combined with the metal in less than the stoichiometric amount normally
present in the metal oxide (i.e., the metal species is partially or
substantially deoxygenated). The reduced form of the metal may consist of
metal or metal suboxides chemisorbed on carbon as well as metal-carbon
intercalate compounds. In other words, the carbon layer is modified to
present active sites which include partially or substantially deoxygenated
metal species that are chemically bonded to the carbon layer. Preferably,
the active sites include substantially deoxygenated metal species, which
are capable of reacting with trace impurities in a gas stream. Examples of
metals or their suboxides which may be employed include manganese,
magnesium, and alkali metals such as lithium, sodium and potassium.
An exemplary technique for depositing the carbon layer on the substrate is
a chemical vapor deposition (CVD) technique via the disproportionation of
carbon monoxide (CO) or the dissociation of other carbon sources such as
various hydrocarbons. The deposition can occur under various reaction
times, temperatures, and gas compositions, the control of which being
within the skill of one in the art. For example, if CO is used as the
carbon source, the disproportionation may be conducted at a temperature
above about 250.degree. C. using a gas mixture of about 5 to 15 percent by
volume CO, 1 to 5 percent by volume hydrogen and the remainder a
relatively inert gas such as nitrogen. Preferably, the disproportionation
is carried out using a mixture of CO and hydrogen, which contains from
about 6% to about 14% hydrogen (by volume), at a temperature of from about
350.degree. C. to about 450.degree. C. The thickness and amount of carbon
deposited may be controlled by adjusting the reaction time, temperature,
gas composition and gas flow rate. For example, good carbon deposition may
be achieved at about 435.degree. C. on a 1.1" diameter filter disk formed
from sintered stainless steel fibers using about an 88/12 (vol/vol)
mixture of CO and hydrogen at a total gas flow rate through the filter
disk of about 500 cc/min.
The formation of the active sites may be carried out by first depositing a
precursor metal species on the carbon layer. The former may include an
oxidized form of the metal and/or a deoxygenated form of the metal. The
precursor metal species may be deposited by using a chemical vapor
deposition technique, such as CVD, e.g., forming a vapor of the precursor
metal species to be deposited and contacting the layer of carbon deposited
on the porous substrate layer with the precursor metal species vapor. The
vapor, for example, may be generated from the metal itself or from a
hydroxide or oxide of the metal. The temperature for deposition depends on
the specific porous substrate and precursor metal species employed and,
typically, is greater than about 300.degree. C.
As an alternate method, the precursor metal species may be deposited from a
mixture which includes a solvent or liquid medium and the precursor metal
species, e.g., a solution or slurry of the precursor metal species in the
solvent or liquid medium. Solutions of metal-bearing compounds, such as
metal hydroxides or oxalates, may be employed to impregnate the carbon
coated substrate with the metal precursor species. For example, if
manganese is used, an aqueous solution of manganese oxalate is suitable
for such purpose.
A preferred method of depositing the precursor metal species is to dissolve
or form a slurry of the precursor metal species in a non-aqueous liquid
medium. Preferably, the non-aqueous solvent is one which may be easily
evaporated without leaving a residue, e.g., a solvent such as anhydrous
liquid ammonia. Exemplary solutions of a precursor metal species in a
non-aqueous solvent include solutions of an alkali metal, such as sodium,
in anhydrous liquid ammonia. If metals, which are insoluble or only
sparingly soluble in a non-aqueous solvent, such as anhydrous liquid
ammonia, are to be used as the precursor metal species, a slurry of a very
fine powder of the metal can be suitable for depositing the metal on the
carbon layer. The solution or slurry of the precursor metal species is
passed through the carbon-coated porous substrate, thereby depositing the
precursor metal species on the carbon layer. Nitrogen or another inert gas
may be used to force the solution or slurry through the porous substrate
or suction may be used to draw the solution or slurry through the porous
substrate.
After passing the solution or slurry through the porous substrate to
deposit the metal precursor species on the carbon layer, the non-aqueous
solvent may be driven off by purging with an inert gas such as nitrogen.
Gradual heating to about 110.degree. C. and, preferably 200.degree. C. or
greater, may facilitate the purging process.
The carbon-coated substrate bearing the deposited precursor metal species
is then heated to form metal species chemically bonded to the carbon
layer, i.e., to form active sites on the carbon layer. The term
"chemically bonded" is intended to include ionic and covalent bonds and
van der Waals forces. In other words, "chemically bonded" includes
absorbed metal and metal suboxides on carbon as well as various
metal-carbon groups and intercalate compounds. Typically, the chemical
bonding is accomplished by heating the substrate to greater than about
300.degree. C., preferably above about 380.degree. C. to about 400.degree.
C., and most preferably above about 500.degree. C., thereby forming the
active sites on the carbon layer. The formation of the active sites may be
carried out by heating the carbon-coated substrate in an inert atmosphere
such as inert or noble gases such as nitrogen, argon, helium, and the
like. If the active sites include lithium, potassium or magnesium species,
argon or helium is preferably used to provide the inert atmosphere. The
presence of the inert atmosphere lessens the opportunity for impurities in
the atmosphere and, in particular, oxidizing impurities, to come into
contact with the medium being activated. If impurities are present in the
gas in contact with the medium during the activation process, the
impurities may interact with and consume active sites. The loss of active
sites may adversely affect the impurity removal capabilities of the
reactive medium. Preferably, the activation process is carried out in an
atmosphere of ultra pure inert or noble gas to avoid the deactivation of
active sites due to trace impurities in the gas. The inert gas may also
include a reducing gas, e.g., hydrogen, to facilitate active site
formation. Preferably, the inert gas employed during the activation
process includes hydrogen in a concentration of at least about 1 percent,
more preferably from about 2 to about 35 percent, most preferably from
about 4 to about 10 percent. In another embodiment, the activation process
may be carried out by heating the substrate to greater than about
300.degree. C. under vacuum.
Typically, the substrate is heated to greater than about 500.degree. C. and
maintained at that temperature for a suitable amount of time to form the
active sites (e.g., about one hour) while being purged with a mixture of a
inert gas preferably having at least about 4 percent hydrogen. This
activates the active sites of the carbon layer by reacting the precursor
metal species with the carbon to form active sites on the substrate, i.e.,
to form metal species chemically bonded to the carbon layer. The metal
activated sites may exist in the reduced form (C-M), as an adsorbed metal,
as a metal suboxide, or in oxidized form (CO.sub.2 M), with the removal of
the oxygen necessary to have the preferred highly reactive reduced form.
Stated otherwise, the metal in reduced form has no, or less than a
stoichiometric amount of, oxygen, i.e., the metal species is partially or
substantially deoxygenated. Preferably, the metal species is substantially
deoxygenated. Preferably, the chemical bond anchors the reactive metal
species to the carbon and the carbon is anchored to the porous substrate.
Thus, because the metal species and carbon are anchored, contamination of
the gas stream during filtration/purification assembly is avoided.
In another embodiment, several different metals can be deposited
sequentially or as a mixture onto the carbon layer to permit removal of
selective impurities as the gas passes through each of the layers of the
filter medium. In still another embodiment, the reactive medium may
include two or more layers of carbon deposited on the exposed surfaces.
Each carbon layer may be individually modified to present active sites.
Such media may be produced by sequentially depositing a carbon layer,
depositing a precursor metal species on the carbon layer and then heating
the medium to form the active sites on the carbon layer. This sequence may
be repeated until the desired number of activated carbon/metal layers has
been produced. Reactive media, which include at least two carbon layers
with each layer presenting active sites, have a number of advantages. Such
multi-carbon layer media have a very high impurity removal capacity. The
active sites on the carbon layers may include more than one metal species.
For example, a reactive medium may include first and second carbon layers
deposited on substantially all of the exposed surfaces of the porous
inorganic substrate. The first carbon layer may be modified to present
active sites which include at least partially deoxygenated first metal
species chemically bonded to the first carbon layer and the second carbon
layer may be modified to present active sites which include at least
partially deoxygenated second metal species chemically bonded to the
second carbon layer. This may permit the use of two metals which require
different deposition methods (e.g., one via CVD and the other via solution
deposition).
The present reactive media are highly effective for removing a variety of
impurities from a gas. For example, a reactive medium embodying the
present invention may be used to remove an impurity, such as oxygen,
water, carbon monoxide, carbon dioxide or methane, from a gas stream,
e.g., a stream of an inert or noble gas such as nitrogen or argon.
The present reactive media may also be used to remove moisture from streams
of reactive gases such as oxygen, silanes, hydrogen chloride and hydrogen
bromide. Typically, reactive media having active sites which include
sodium or magnesium species chemically bonded to the carbon layer, may be
employed for this purpose. Even though the reactive gas may react with the
active sites, moisture may still be chemisorbed by the reactive medium
(i.e., the water interacts with the active sites, thereby substantially
removing the water from the reactive gas). For example, although HCl or
HBr may react with an active site (e.g., active sites which include
magnesium species) to form a metal halide, moisture may still be
chemisorbed by the metal halide to form a hydrate. Similarly, oxygen may
react with the metal species of the active sites to form an oxidized site.
Since the oxidized sites are capable of interacting with moisture, the
reactive media of the present invention may be employed to remove trace
levels of moisture from a reactive gas like oxygen. Reactive media
embodying the present invention may also be used to remove oxygen from
silanes.
In operation, a gas stream including impurities, is passed through
filtration/purification assembly of the invention including the reactive
filter medium, which medium may have any one of a variety of known
configurations. It is recognized that a liquid stream could also be
filtered/purified by altering the medium in a manner known to those
skilled in the art. Heterogeneous particles are removed from the gas
stream by well known filter mechanisms. The active sites of the medium
interact, and preferably react, with trace homogeneous impurities present
in the gas stream (e.g. oxygen, carbon dioxide, carbon monoxide, water,
organic compounds and the like) thereby removing the homogeneous
impurities from the gas stream. Although not wishing to be bound by any
theory, it is thought that substantially all of the impurity molecules
come in contact with the metal molecules of the reactive layer, interact,
and are removed by the medium from the gas being filtered and purified. In
other words, the impurities interact with the metal species which are
bonded to the active sites of the carbon layer, thereby scavenging and
removing the impurities from the gas phase.
Once purification is complete and all of the reduced metal is oxidized or
otherwise deactivated by contaminants, the medium may be regenerated by
heating the substrate to greater than about 300.degree. C. in an inert
atmosphere (e.g., nitrogen or argon) to reduce the oxidized metal (i.e.,
regenerate the active sites). Alternatively, the regeneration may be
carried out by heating the deactivated reactive medium to greater than
about 300.degree. C. under a hydrogen-containing atmosphere. The
regeneration capability of the reactive medium, a process which may be
repeated several times, provides a significant advantage over known
filtration/purification assembly materials which typically may be used
only once and for which there are apparently no known means of
regeneration. Regeneration is a feasible option with those embodiments of
the invention in which, as necessary, the reactive filtration/purification
assembly may be disassembled to provide access of the media to carbon
coating and to remove valve type barrier members from exposure to high
temperatures and contact with carbon or carbon-producing materials.
The medium within the filtration/purification assembly of the invention is
typically regenerated after use by heating the housing portion containing
the medium to at least about 300.degree. C., preferably to greater than
about 450.degree. C., most preferably to a temperature of about
450.degree. C. to about 550.degree. C., in an inert or noble gas
atmosphere (e.g., nitrogen, argon, helium and the like). The carbon layer
may participate in regeneration of the active sites in that, on heating in
an inert atmosphere, the carbon may function as a reducing agent. The
regeneration process is typically carried out for a time period of about
24 to 28 hours, although longer time periods, e.g., about 48 hours, may
also be employed.
Alternatively, the medium within the filtration/purification assembly of
the invention may be regenerated after use by heating the medium to
greater than about 300.degree. C., and preferably to a temperature of
about 450.degree. C. to about 550.degree. C., in the presence of a
reducing atmosphere, such as a hydrogen-containing atmosphere. For
example, the medium may be regenerated by heating to greater than about
500.degree. C. and maintaining the medium at that temperature for about
one hour while it is purged with a mixture of hydrogen and an inert gas.
Preferably, the regeneration process is carried out using an atmosphere of
an inert gas having from about 2 percent to about 100 percent hydrogen
(vol:vol), more preferably from about 10 percent to about 40 percent
hydrogen. Typically, the regeneration of the reactive medium is carried
out for a time period of from about 8 to about 28 hours under these
conditions. As with the activation process used to originally form the
active sites, the regeneration may also be carried out under an inert gas
containing a lesser amount of hydrogen. With lower concentrations of
hydrogen, longer times and/or higher temperatures are typically employed
for the regeneration process. For example, the regeneration of a stainless
steel reactive medium may be carried out by heating the medium for 12 to
24 hours at about 500.degree. C. while purging with a 35:65 (vol:vol)
mixture of hydrogen and nitrogen.
Assembly of the device according to the present invention may be
accomplished by various methods. The manner in which the devices of the
present invention are assembled depend, in part, on the types of barrier
members employed and the particular configuration of the housing. In most
embodiments of the invention it is preferred that the individual
components, i.e., reactive filter, barrier members, etc., be located in
the appropriate section of the housing and, as appropriate, thereafter
treated to put the active carbon and metal coatings thereon. In those
instances in which another type of barrier member is employed, such as a
valve, the device is partially assembled with the housing portion
containing the metal filters coated with a carbon layer and containing
active metal species formed separately from the housing portion containing
a valve and the two housing sections are thereafter joined. The exact
order in which the barrier member(s) and reactive filter medium are joined
to or within the housing section(s) depends in part on the manufacturing
and assembly facilities. Typically, the downstream barrier member, such as
a small reactive type filter is fixedly positioned in the outlet which
communicates with the housing, transverse to the downstream opening of the
outlet such that all fluid passing out of the housing during operation, or
into the downstream side of the housing immediately prior to placing the
filter unit into service, must pass through the reactive type filter.
The manner of permanently locating the reactive type filter in the outlet
could be accomplished by swaging, staking, brazing, sintering or
press-fitting of the barrier member at the appropriate location.
Thus, by way of example, in the embodiments illustrated in FIGS. 2 and 3, a
metal filter element, such as a precursor substrate to a reactive filter,
i.e., prior to carbon coating and bonding of metal species, may be
located, fixedly or removably, at the end of the downstream outlet 19. The
outlet may be formed as part of a downstream housing section 15a or 15a'
or may be attached to section 15a or 15a' by methods such as welding,
either before or after the downstream barrier member is fixedly positioned
in the outlet. The principal reactive metal filter which is the precursor
or substrate prior to coating to form the fluid permeable, reactive medium
filter 13 (in FIGS. 2 and 3) may then be located in the housing by
removably or fixedly securing the metal filter substrate to either or each
of housing sections 15b (or 15b') and 15a (or 15a'), with or without the
outlet 19 joined thereto. Typically housing element 15b (or 15b'),
reactive filter medium 13 and housing member 15a (or 15a') are joined by
welding, brazing or similar technique. In the embodiment illustrated in
FIG. 3, the metal filter substrate, which ultimately becomes a barrier
member 21', which is the same as or substantially similar to the
downstream barrier member 23 or 23', is fixedly or removably positioned in
the inlet 17'. This is done preferably at the entrance or upstream end of
the inlet 17' in the same manner as is done at the outlet 19. As with the
outlet 19 in housing section 15a, the inlet 17' and housing section 15b'
may be attached to one another either before or after the metal filter
substrate which forms reactive medium filter 21' is fixedly positioned in
the inlet 17'.
In the embodiment of the type illustrated in FIG. 3, in some instances the
upstream housing section(s), inlet and filter member may have a similar or
identical configuration to the downstream housing section(s), outlet and
filter member. Costs of manufacturing can be saved when the upstream and
downstream portions of the assembly are identical.
The metal filter substrates, positioned within the designated portions of
the assembly, may be more efficiently coated with carbon and activated
with metal species by positioning the metal filter substrates in the
appropriate housing section(s) prior to the treatment steps and thereafter
assembling the separate housing sections. Thus, in the embodiment
illustrated in FIG. 2, the outlet 19 and housing sections 15a and 15b,
containing the metal filter substrate which forms the reactive filter
medium 13, are separately assembled and treated to produce carbon coatings
bonded to active metal species prior to joining the upstream housing
section 15c and the inlet 17 containing the poppet valve 21. The upstream
housing section 15c may be joined by welding or the like to housing
section 15b. A similar procedure may be used with the embodiment of FIG.
3.
Coating of the metal filters and activation with metal species to form the
reactive filter medium and barrier member(s) of the reactive filter type
may be performed in the manner indicated above and in U.S. Pat. No.
5,637,544. Alternatively, the following modified procedure may be
employed. The filtration/purification assembly or portion thereof
containing metal filter substrate(s) may be initially subjected to a
treatment with a reducing agent. Thus, the portion of the filter assembly
containing the metal filter substrates may be connected to a manifold
which is also in fluid communication with sources of an inert gas or a
noble gas, such as argon, helium and in some instances nitrogen, as well
as hydrogen and carbon monoxide. Initially, to purge air and atmospheric
contamination from the assembly, the inert gas, such as argon, is supplied
to the housing at a rate of 0.7 to 1.2 cc/min-mm.sup.2, typically at or
slightly above ambient temperature. Thereafter, in order to strip the
surface of the naturally occurring mixed oxides present on the surface of
metals, such as iron and chromium oxides, typically present on a stainless
steel surface, and form a reduced metal surface, argon flow is terminated
and the system is purged with purified hydrogen at a rate of about 0.8 to
about 1.5 cc/min-mm.sup.2. The purging is continued at ambient temperature
for approximately 10 to 20 minutes or until hydrogen completely displaces
argon from the system. The temperature is then increased to about 400 to
about 450.degree. C. at a rate of about 2 to 8 C..degree./min. Hydrogen
flow is allowed to continue as the filter is maintained within this
temperature range for about 3 hours to from a uniformly reduced or
elemental metal surface. Thereafter, an inert or a noble gas, such as 100%
argon, is passed through the filtration/purification assembly at a flow
rate of about 2.2 to about 2.6 cc/min-mm.sup.2. Argon flow is continued
through the assembly and the temperature is permitted to drop to about 300
to about 350.degree. C.
A controlled oxidation step is then conducted which renders a relatively
thick and uniform oxide on the surface of the metal which consists
primarily of iron oxide when a metal such as stainless steel is employed.
This uniform iron oxide layer acts as a catalyst to promote uniform
dissociation of carbon monoxide and deposition of elemental carbon on the
substrate surface. This also has the effect of increasing the number of
active metal sites on the carbon of the reactive filter element. The
oxygen is introduced to the assembly to provide an oxygen and argon
mixture containing about 35% to about 45%, preferably about 40% oxygen
with a flow rate of about 2.2 to about 2.6 cc/min-mm.sup.2. Flow of this
oxygen/argon mixture is continued within the range of about 300 to about
350.degree. c. for a period of about 2 to about 4 hours, preferably about
2.5 hours. Thereafter, the furnace in which the reactive filter element is
heated is turned off and 100% argon is passed through the assembly at a
flow rate of about 2.2 to about 2.6 cc/min-mm.sup.2 for a time sufficient
to allow the uniformly oxidized substrate to cool to or close to room
temperature if carbon coating is not begun immediately thereafter.
Subsequently, as described above, the oxidized metal filter substrate is
coated with carbon and active metal species are introduced to the carbon
with a vapor deposition process or a solvent-metal procedure as described
above.
As a result of the procedure of reducing and subsequently providing a
uniform oxidized surface to the metal filter substrate or element, the
ratio of the total surface area of the carbon coating to the total surface
area of the metal filter substrate is about 25 to 65 m.sup.2 /m.sup.2 as
compared to 2.3 to 3.6 m.sup.2 /m.sup.2 without oxidation pretreatment.
This also produces a ratio of sodium metal coating (when the carbon is
activated with sodium metal species) to surface area of carbon coating of
about 1.2 to about 1.6 mg Na/m.sup.2 carbon. The resultant oxygen capacity
is about 4 to about 6 times greater than without oxidation pretreatment.
EXAMPLE
An example of a specific method of preparing the housing or housing
portions containing metal filter substrates prior to coating with carbon
and bonding metal species to active sites on the carbon, such that the
metal substrates have a uniform oxide coating may be performed as follows.
A housing section containing a metal filter substrate welded in place
across a fluid flow path was attached to a manifold that was in turn
connected to sources of several different pure gases. The housing section
was supported within a furnace and was initially purged of atmospheric
contamination with purified argon at a flow rate of 500 cc/min for
approximately 10 minutes at ambient temperature. Thereafter, the flow of
argon through the housing section was discontinued and the housing section
was purged with purified hydrogen at a flow rate of 600 cc/min, also at
ambient temperature, for a period of about 10 to 20 minutes. The
temperature of the housing section containing the metal substrate was
gradually increased to 425.degree. C. at a rate of 2 C..degree./min. The
housing section was maintained, with hydrogen flowing at a rate of 600
cc/min, at 425.degree. C. for three hours. Thereafter, hydrogen flow was
discontinued and pure argon was passed through the housing section at a
flow rate of 1500 cc/min. With argon flowing through the housing section
at the same rate, the temperature of the housing section was allowed to
decrease to 305.degree. C. Thereafter, oxygen was mixed with argon to
provide a gaseous mixture flowing through the housing section of 40%
oxygen and 60% argon at a flow rate of approximately 1500 cc/min. This was
continued for a period of 2.5 hours at a temperature of 305.degree. C.
Thereafter, the furnace was turned off and 100% argon was passed through
the housing section at a rate of 1500 cc/min. Carbon deposition and
activation of sites on the carbon deposit were performed as indicated
above.
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